As an example of the relationship between stress and depression, J. Raymond DePaulo, Jr., M.D. mentions the situation with CBS commentator Mike Wallace, when he was on trial for libel over a story he’d done. In “Understanding Depression,” DePaulo writes:

For weeks [Mike Wallace] sat in a drafty federal courthouse in New York City while the prosecution denounced him as a liar and a fraud. So naturally, he thought, he would be feeling down. Who wouldn’t in such a situation? “I’m pessimistic by nature and so it didn’t occur to me when I began to feel badly when I was on trial for libel,” he recounted at an annual gathering of DRADA, the support group for patients and their families, at Johns Hopkins. “I felt lower than a snake’s belly. I used to speak to my own physician and we’d known each other for ten or fifteen years. He said, ‘You’re strong, you’re talking yourself into something.’ ‘No, I’m not,’ I told him. Finally I wound up collapsing and going into Lenox Hill Hospital.”

Although Wallace was ultimately vindicated, the ordeal was so agonizing for him—his professional reputation was on the line—that you might assume that the court battle was in some way responsible for “causing” the depression. It certainly appears to have been a precipitating factor. So you could say that this case of depression was essentially externally influenced.
However, when Wallace next suffered an episode of depressive illness several years later, the only significant development that preceded it was a fall while playing tennis that resulted in a broken wrist. It certainly isn’t in the same ballpark with being sued for $120 million for libel. A broken wrist may be cause for some demoralization—you aren’t going to be able to play tennis or piano for some time—but on its own would hardly seem to account for a depression that requires hospitalization. What we can safely conclude is that environmental influences are important in both depression and demoralization, but they do not provide a sufficient explanation for major depressive illness or manic depression even in the direst circumstances.